High molecular weight Mannich polyacrylamides (PAMS) are well known and are used in a variety of flocculant applications. A major drawback of Mannich PAMS is their tendency to cross-link. This problem becomes more severe as the polymer solids are increased. Consequently, these polymers are generally made as dilute, aqueous solutions in an attempt to slow down the rate of interpolymer cross-linking. The solids level must also be kept low, particularly for very high molecular weight Mannich PAMS, owing to the ability of these polymers to viscosify water. As a result, solids levels of very high molecular weight Mannich PAMS must generally be well below 10%, and more typically 6% or less so that the solutions can be pumped and handled conveniently.
Several approaches have been tried to overcome these problems. One approach has been to make the Mannich PAMS at the site of use by inverting high solids inverse emulsion PAMS in water containing dialkylamines and formaldehyde. U.S. Pat. Nos. 4,021,394 and 4,022,741 describe continuous processes for the preparation of Mannich PAMS which entails inverting an inverse emulsion PAM in a process stream containing formaldehyde and a secondary amine and subjecting the stream to turbulence by in-line mixing to produce a 1-15% aqueous solution of Mannich PAM. This approach, however, suffers from the need to store multiple chemicals on site and from the problems inherent in running chemical reactions at such locations. Another approach has been to prepare dry Mannich PAMS, as described in U.S. Pat. Nos. 3,864,312; 3,539,535 and 3,790,529 or blends of dry PAMS with dry, low-molecular weight Mannich-base forming compounds which, when dissolved in water, react to produce Mannich PAMS, as described in EPO Patent No. 0,210,784. These approaches, in general, suffer from cross-linking problems, the reversibility of the Mannich reaction, the difficulty and length of time required to dissolve high molecular weight polymers, and other problems. Another approach has been to make the Mannich PAM in inverse emulsions, such as described in U.S. Pat. Nos. 3,979,348; 4,093,542 and 4,010,131. While this approach produces a product with substantially higher solids, the average particle size thereof ranges from about 10,000-20,000.ANG. in diameter, and consequently, cross-linking of the many polymer chains in each particle renders the products less effective. The cross-linking rate can be reduced somewhat by adding fairly large quantities of stabilizers, such as described in U.S. Pat. Nos. 4,113,685 and 4,073,763, but cross-linking continues and the products thus possess a very limited shelf life.
Accordingly, there exists a need for a Mannich acrylamide polymer which can be prepared at high solids levels without extensive interpolymer cross-linking such that it may be economically transported and easily handled by the end user without the need for any on-site preparation. Such a Mannich acrylamide polymer composition would satisfy a long felt need and constitute a notable advance in the art.
Surprisingly, it has now been discovered that Mannich acrylamide polymers, in the form of inverse microemulsions, give superior performance relative to the Mannich acrylamide polymers of the prior art and can be conveniently prepared at high solids content while maintaining a very low bulk viscosity. In contrast to solution and inverse emulsion Mannich acrylamide polymers of the prior art which contain large quantities of polymer molecules in the same aqueous environment, the Mannich acrylamide polymers of the present invention are isolated as individual, or at most, several, polymer molecules in each aqueous microemulsion micelle. Thus, the problem of large scale debilitating interpolymer cross-linking inherent in the solution and inverse emulsion products of the prior art is overcome.
Additionally, in contrast to the high bulk viscosities of the more stable dilute solution acrylamide polymers of the prior art, the novel microemulsion Mannich acrylamide polymers of the present invention can be made at high solids levels while still maintaining an extremely low bulk viscosity.
Even more unexpected with regard to the microemulsion Mannich acrylamide polymers of the instant invention is the fact that even though cross-linking thereof still occurs, the cross-linked polymers are capable of successfully performing as effective flocculants in the dewatering of suspended solids, whereas the cross-linked inverse emulsion and solution Mannich acrylamide polymers fail.